The present invention relates to a propeller blade pitch actuation system. In particular, the invention relates to increasing a pressure force available for changing the pitch of a propeller blade.
Propeller driven aircraft typically include a system for adjusting the pitch of each propeller blade. Blade pitch generally ranges from a fine pitch position, where the blade has a low level of attack, to a coarse pitch position, where the blade has a high angle of attack. In the fine pitch position, the blade moves relatively little air with each rotation, permitting operation when little forward thrust is desired. When more thrust is desired, the blade pitch becomes coarser, moving more air with each rotation. Increased engine power and gears between the engine and the propeller provide the increased torque necessary to keep the propeller moving at a desired speed as the blade pitch becomes coarser. At the extreme coarse pitch position, or feathered position, the blade is pitched in the direction of flight to minimize propeller drag in the event of engine failure. At the other extreme, the blade is pitched past the fine pitch position to a negative angle of attack to produce reverse thrust. Reverse thrust may be employed to slow the aircraft once it has landed. In either forward or reverse thrust conditions, as the blade pitch moves away from a fine pitch position, increasingly greater force is required to adjust the blade pitch.
Generally, blade pitch changes are driven by a propeller actuator connected by a trunnion pin to each blade. The propeller actuator is a translating piston which moves when a pressure force, in the form of a hydraulic pressure difference, is applied to the piston. The translation of the piston is transmitted through the trunnion pin to the blade which rotates on a bearing assembly. Providing a pressure force sufficient to move the blade pitch to full coarse and full reverse positions under desired operating conditions often requires a high-pressure hydraulic pump to provide the necessary hydraulic pressure. In addition, a propeller actuator, hydraulic lines and fittings must be strong enough to withstand the hydraulic pressure. Increasing the pressure force by increasing the hydraulic pressure requires increasingly heavy or expensive pumps, pressure actuators, lines, and fittings. This is of particular concern on aircraft, where weight is a key factor in the economical operation of the aircraft. Thus, there is a need to provide a pressure force sufficient to move the blade pitch throughout its range under desired operating conditions, while reducing the cost or weight penalty of the high-pressure pump, pressure actuator, lines, and fittings.